1. Field of the Invention
The present invention relates to an image processing device, a radiography system, an image processing program, and an image processing method, and more particularly, to an image processing device, a radiography system, an image processing program, and an image processing method which generate a tomographic image from projection images that are captured by emitting radiation at different incident angles.
2. Description of the Related Art
A radiography apparatus is known which captures radiological images for medical diagnosis. An example of this type of radiography apparatus is a mammography apparatus which captures an image of the breast of a subject for the early detection of breast cancer. In addition, in mammography, a tomosynthesis imaging technique is known which irradiates the breast of the subject with radiation at different angles to capture images of the breast. The tomosynthesis imaging technique irradiates a subject that is positioned between a radiation detector and a radiation emitting unit and of which the tomographic image is to be generated with radiation at different incident angles (hereinafter, simply referred to as “incident angles”) with respect to a direction normal to a tomographic plane of the tomographic image in a predetermined range while moving the radiation emitting unit, captures images of the subject at different incident angles, and reconstructs a plurality of captured radiological images (hereinafter, referred to as “projection images”) to generate tomographic images at a predetermined slice interval.
In mammography diagnosis, tomosynthesis imaging has come into widespread use. However, in many cases, tomographic images obtained by the tomosynthesis imaging are used to supplement radiological images obtained by general two-dimensional imaging which irradiates a subject with radiation at a fixed position, without moving a radiation emitting unit, and captures images of the subject. The reason for this is that the radiological image obtained by general two-dimensional imaging has concentration that a doctor is accustomed to seeing and is different from that of the tomographic image and the doctor can check the entire radiological image at one time.
For this reason, in many cases, both the two-dimensional imaging and the tomosynthesis imaging are performed and diagnosis is performed using a combination of the radiological image obtained by the two-dimensional imaging and the tomographic image obtained by the tomosynthesis imaging.
However, if an image corresponding to the radiological image obtained by the general two-dimensional imaging can be obtained by only the tomosynthesis imaging, it is possible to significantly reduce a radiation dose and an imaging time during imaging, which is preferable.
As a technique which generates an image corresponding to the radiological image obtained by the general two-dimensional imaging from the tomographic image obtained by the tomosynthesis imaging, US2010/0135558A discloses a method which captures a plurality of X-ray tomosynthesis projection images of the breast of a patient and combines at least one sub-set of a plurality of X-ray tomosynthesis projection images to synthesize two-dimensional mammograms, using at least one of an algebraic method and a maximum intensity projection method.
Hereinafter, a two-dimensional image, such as a two-dimensional mammogram generated by combining a plurality of tomographic images, is referred to as a “composite two-dimensional image”.